As a vacuum carburizing method of performing a carburizing process on steel parts for automobile such as gears, bearings, fuel injection nozzles and constant velocity joints, for example, a method of using ethylene gas as a carburizing gas to perform the process under reduced pressures of 1 to 10 kPa in a vacuum heat treating furnace has been known (see Japanese Unexamined Patent Publication No. 11-315363).
In the conventional method, however, when the vacuum carburization is performed while disposing a basket which carries a number of objects to be treated (workpieces) in an effective heating space where uniformity of temperature is ensured in the vacuum heat treating furnace, there arises a problem that unevenness of carburization occurs in the workpieces depending on the carried position in the basket, and variation occurs in carburization quality such as effective case depth (carburization depth) and surface carbon concentration among workpieces at different carried positions.
Thus, as a vacuum carburizing method which solves the above described problem, the present applicant has previously proposed a method of using a mixed gas of ethylene gas and hydrogen gas as a carburizing gas (see Japanese Unexamined Patent Publication No. 2001-262313).
In the vacuum carburizing method previously proposed by the present applicant, even when carburization is performed while disposing a number of workpieces in an effective space where uniformity of temperature is ensured in the vacuum heat treating furnace, it is possible to prevent unevenness of carburization from occurring in all workpieces, so that carburization quality of all the workpieces can be made uniform.
In this method, however, a technique capable of obtaining the material (specification) and required carburization quality of the workpiece with accuracy and reproducibility has not been established.
The present invention has been made in consideration of the above described current condition, and it is an object of the present invention to provide a vacuum heat treating method and apparatus therefor capable of obtaining heat treatment quality which is required for a workpiece with accuracy and reproducibility in a method disclosed in Japanese Unexamined Patent publication No. 2001-262313.
It is another object of the present invention to provide a vacuum heat treating apparatus capable of readily setting heat treating condition in accordance with the material, shape of the workpiece, ventilation condition when workpieces are loaded in a processing basket, and required heat treatment quality.